"Linphone makes use of the SIP protocol (...) You can use Linphone with any SIP VoIP operator, including our free SIP audio/video service. (...) Linphone is available for desktop computers: Linux, Windows, MacOSX, and for mobile phones: Android, iPhone, Blackberry."

"Jitsi is an audio/video Internet phone and instant messenger written in Java. It supports some of the most popular instant messaging and telephony protocols such as SIP, Jabber/XMPP (and hence Facebook and Google Talk), AIM, ICQ, MSN, Yahoo! Messenger."
Not available for iOS.